Investigation of synergistic antimicrobial effects of combined dentifrice components

'Pluralibacter gergoviae' ('P. gergoviae') is a Gram negative, facultative anaerobe of the 'Enterobacteriaceae' family; it is a common contaminant of water, the immunocompromised, and a number of cosmetics. This project aimed to identify the ways in which the robustness of a cosmetic product, namely dentifrice, could be improved without the inclusion of parabens of preservatives as 'P. gergoviae' has exhibited resistance against parabens. In order to achieve this minimum inhibitory concentration (MIC) and checkerboard testing were carried out on 17 dentifrice ingredients to identify potential antimicrobial and synergistic activity. From this, eleven antimicrobial excipients were identified including glycerol (MIC 25% w/w), zinc gluconate (MIC 0.15% w/w) and tego betaine (MIC 11.75% w/w). After 75 checkerboard tests 5 synergistic interactions were identified, using the Fractional inhibitory concentration index criteria, however many interactions could not be classified. Antimicrobial efficacy testing was then carried out on 34 formulations with adjusted concentrations of the ingredients identified from Mic as being antimicrobial. Formulations with elevated humectant ([greater than or equal to] 12.5% w/w glycerol or [greater than or equal to]1% w/w PEG300), and adinol or tego betaine ([greater than or equal to] 0.825% w/w) were more robust against 'Pluralibacter gergoviae', meeting the criteria for antimicrobial efficacy outlined by the pharmacopoeia. This body of works highlights some options for dentifrice formulation changes that can be applied across all formulations, without resorting to preservative systems against which 'P.gergoviae' may develop resistance.